"The problem right now is that it's so inaccessible," Judy said of the blaze.

He said crews on the ground were struggling to cut containment lines and douse flames in "steep, nasty terrain" with slopes ranging from 40 to 80 degrees.

"The slope up there is just incredible," he said in a telephone interview from the fire area.

About 400 firefighters were battling the blaze, which was about 5% contained. In the air, six tanker planes and eight helicopters were making repeated water and fire-retardant drops on the blaze.

Flames were between the 2,500- and 3,000-foot elevation level in medium-to-heavy brush that has not burned in about 15 years, officials said.

The fire broke out Sunday afternoon about 3-1/2 miles east of California 39 and prompted officials to evacuate visitors and residents from campgrounds, picnic areas and a mobile home park at Camp Williams on the east fork of the San Gabriel River. The facilities typically have up to 12,000 visitors during the Labor Day weekend.

On Monday, the entire San Gabriel Canyon was shut down by authorities, who opened an evacuation center at Glendora High School at 1600 E. Foothill Blvd.

No injuries were reported, officials said, and the cause was under investigation.